Sudanese soldiers stand to attention during President Omar al-Bashir's visit to the Popular Defence Forces in Khartoum on March 3, 2012. (Ebrahim Hamid /AFP/Getty Images)

NAIROBI, Kenya — The two Sudans have been accusing each other of backing rebels on either side of the disputed oil-rich border that since July has separated north from south, but on Monday their armies clashed directly.

The North accused the southern army, the SPLA, of occupying the Heglig oil field which both Juba and Khartoum claim. The SPLA said its occupation of Heglig was "a self-defence measure."

This week's direct clashes immediately worsened the already poisonous relations between North and South who are gridlocked over a series of issues including border demarcation and the sharing of oil revenues.

Tracking Africa from the Sahara to Cape Town, to everything in between. The politics, economics and culture of a continent. On Africa is a reported blog from Nairobi, written by GlobalPost senior correspondent for Africa Tristan McConnell.